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We report on ultraviolet spectra of the M8~Ve star VB10 = Gl 752B, probably the coolest and lowest mass star observed so far in the ultraviolet. This star is of great interest because it lies almost at the end of the main sequence where stars are thought to be fully convective and solar-type dynamo processes should not be present. On 1994 October 12 we observed the brighter companion Gl 752A (M3~Ve) and then offset to VB10. Both stars were observed with the G140L grating on the HST Goddard High Resolution Spectrograph. The spectrum of Gl 752A shows the expected transition region lines of solar-type stars consisting of C~III 1175\AA, H~I Lyman-$\alpha$, N~V 1240\AA, O~I 1304\AA, C~II 1335\AA, Si~IV 1400\AA, C~IV 1550\AA, He~II 1640\AA, and others.
The spectrum of VB10, on the other hand, provided a surprise. Our spectra of this star consists of 11 integrations, each of about 5 minutes duration. The first 10 integrations show no emission features with very small upper limits to the surface fluxes in the transition region lines. The last integration, however, shows strong emission in the C~II, Si~IV, and C~IV lines, which we interpret as a flare.
The VB10 spectra imply that there is little if any continuous heating of the transition regions of the very coolest M dwarf stars. Instead, there is only transient emission during major realignments of the magnetic field. By contrast, hotter stars show continuous emission in the transition region lines, indicating a continuous heating process or a large number of small flares (microflaring). This change in behavior may be due to the absence of radiative cores in the coolest M dwarfs and the inability of the solar-type $\alpha$-$\omega$ dynamo to operate in stars without an interface between a radiative core and a convective envelope. Our data indicate that the coolest M dwarfs nevertheless do have magnetic fields.
This work is supported by NASA Interagency Transfer S-56460-D to the National Institute of Standards and Technology.
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